The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for zero voltage switching. Merely by way of example, the invention has been applied to a power conversion system. But it would be recognized that the invention has a much broader range of applicability.
Modern society faces challenges arising from excessive energy consumption and severe environmental damages. It is highly desirable to promote energy saving and emission reduction. In order to improve energy efficiency of electrical equipments and electronic products, power conversion systems can be optimized to increase power conversion efficiency and lower static stand-by power consumption. In medium-low power applications, the fly-back topology has become one of the most widely used topologies because of its many advantages, such as simple structures, low cost, wide input/output voltage ranges, and small sizes.
Fly-back power conversion systems often implement different operational modes. For example, a fly-back power conversion system can operate in a continuous conduction mode (CCM) and/or a discontinuous conduction mode (DCM). However, the power conversion efficiency of such a power conversion system usually decreases as the operating frequency increases. The switching loss may become a significant problem for a high-density small size switching power supply. In another example, a fly-back power conversion system can operate in a critical conduction mode (CRM) or a quasi-resonant mode (QR).
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system. The power conversion system 100 includes a transformer 102, resistors 104 and 106, capacitors 108, 110 and 114, diodes 116, 118 and 128, a switch 112, and a controller 120. The transformer 102 includes a primary winding 122, a secondary winding 124, and an auxiliary winding 126. For example, the switch 112 is a bipolar junction transistor, a field effect transistor, or an insulated-gate bipolar transistor. The switch 112 includes two terminals 136 and 138. As an example, the primary winding 122 includes a parasitic capacitor 160. In another example, the switch 112 includes a parasitic capacitor 134.
The power conversion system 100 uses the transformer 102 as an inductor capable of storing energy. Also, the transformer 102 isolates an input voltage 130 on the primary side and an output voltage 132 on the secondary side. Thus, the flyback power conversion system 100 usually does not need an output inductor as in a forward structure.
FIG. 2 is a simplified conventional timing diagram for the power conversion system 100 operating in the critical conduction mode (CRM). For example, the critical conduction mode (CRM) is the quasi-resonant mode (QR). The waveform 202 represents a voltage drop of the switch 112 (e.g., the voltage difference between the terminal 136 and the terminal 138) as a function of time, and the waveform 204 represents a current flowing through the primary winding 122 as a function of time. As shown in FIG. 2, during the time period between time t1 and time t3, the switch 112 is open (e.g., off), and during the time period between the time t3 and time t4, the switch 112 is closed (e.g., on). For example, t1≦t2≦t3≦t4.
However, the power conversion system 100 has some disadvantages. For example, when the switch 112 is turned off, the switch 112 may sustain a high voltage stress. The leakage inductance energy often has to be absorbed by one or more extra circuit components, such as the capacitor 110, the resistor 104 and the diode 116. The switching circuit components (e.g., the parasitic capacitor 134 and the inductance of the primary winding 122) often generate resonant waves which may affect electro-magnetic interference (EMI) of the power conversion system 100. In addition, the switching loss wastes energy and may generate excessive heat so as to negatively affect the system safety.
Hence, it is highly desirable to improve techniques for efficiency of switching power conversion systems.